Subterranean boring machines are used to install a casing or pipe in the ground without excavating a trench for the casing. The boring machine generally includes a sled that rolls along a track comprised of a pair of track rails, which track is generally placed in a pit that is dug to a depth to permit the sled to be placed in alignment and on grade with the desired underground installation. A section of casing is located on the front end of the sled with a cutting head or auger mounted thereon. The sled carries a rotation mechanism for rotating the auger and a translation mechanism for driving the sled along the track so as to drive the auger section into the ground as it rotates, along with a surrounding casing section. Generally, the translation mechanism includes a pair of dogs which engage drive holes in the track rails and a pair of hydraulic actuators. In operation, the dogs engage a set of drive holes and the hydraulic actuators are extended to drive the sled in the boring direction. When the actuators have extended to their maximum length, the dogs are disengaged from the track rails and the actuators are fully retracted. Then the dogs engage a second set of drive holes and the actuators are extended to drive the sled another step in the boring direction. This incremental driving process is continued as the sled travels to the terminal end of the track. Once the sled has reached the terminal end of the track and has driven an auger section and a casing section into the ground by the distance of its travel, the casing and auger sections are released from the sled and the sled is retracted from the terminal end back to the initial end. Sections of casing and auger are then added to the ends of the casing and auger sections that protrude from the bore, and the incremental driving process is repeated until enough sections of the casing have been driven into the ground to comprise the desired overall length of the subterranean installation. Once all of the sections of casing are installed, the auger sections must be removed from the casing sections and, unless the casings are installed merely for drainage, an underground utility product must then be placed within the casings.
In most conventional auger boring machines, the translation mechanism that is employed to move the sled in the forward direction also moves the sled in the reverse direction on the track, employing the same hydraulic actuators that are used to drive the sled forward (in the boring direction). In this conventional reverse translation process, the actuators are repeatedly extended and retracted in conjunction with the incremental engagement and disengagement of the dogs in the drive holes. That is, the dogs are retracted from a pair of drive holes and the actuators are extended to drive the dogs in the rearward direction until they are aligned with the previous set of drive holes. The dogs are then engaged with the drive holes and the actuators retracted to move the sled in the rearward direction. This repeated extension and retraction process is continued until the sled reaches the initial end of the track. Since this process for moving the sled in the reverse direction on the track employs the same hydraulic actuators and dogs as are used in moving the sled in the boring direction, movement of the sled in the reverse direction, whether to move the sled back to receive a section of casing and auger, to withdraw an auger section or for any other purpose, will generally take as much time as it takes to move it in the boring direction.
Another known method for moving the sled in the reverse direction on the track may be employed when the sled is equipped with a power winch. In the practice of this method, a wire rope is extended from the winch and attached to a fixture at the initial end of the track, and the winch is used to pull the sled back from the terminal end. This method may be faster than the incremental method described above; however, it is generally only suitable for moving the sled back to receive a section of casing and auger for further boring.
It is known to provide a supplemental drive system for a subterranean boring machine, which supplemental drive system may be used to move the sled in the reverse direction more quickly than the conventional drive system. Thus, for example, U.S. Pat. No. 6,374,929 and No. 6,715,565 of Barbera both describe a supplemental drive system which includes a primary and a secondary drive wheel on each side of the sled. A drive sprocket is attached to the primary drive wheel and the primary drive wheel is mounted on the shaft of a hydraulic motor. An idler sprocket is mounted on the secondary drive wheel, and a chain connects the drive sprocket and the idler sprocket. Each supplemental drive system is mounted so that the drive wheels are biased against the track by a pair of springs to cause the primary and secondary drive wheels to frictionally engage the track. The drive motor drives the primary drive wheel, which in turn, drives the secondary drive wheel so as to move the sled along the track when the sled is not driving an auger section and surrounding casing section into the ground. The Barbera system may be subject to slippage if oil or water is introduced on the track or if its springs do not provide sufficient biasing force to ensure that the wheels frictionally engage the track. Furthermore, it is believed that the Barbera system for frictional engagement does not have the power to withdraw auger sections from the installed casings.
U.S. patent application Ser. No. 10/886,808 describes an auger boring machine for use in connection with a track having a gear rack. The preferred embodiment of this auger boring machine includes a sled that is mounted on the track and adapted to be moved by a drive mechanism which includes a drive motor having a shaft and a pinion mounted on the shaft which is adapted to engage with the gear rack on the track to drive the sled along the track. In the preferred embodiment of this auger boring machine, rack-and-pinion drive mechanism is a supplemental drive system employed to move sled in the rearward direction, and the sled is also provided with a conventional translation mechanism that drives the sled along the track while the cutting head is boring the bore for the casing. The preferred embodiment of this auger boring machine also includes a rack-and-pinion drive mechanism for each side of the sled (and for each rail of the track). In various embodiments of the invention, the drive motor may be arranged to rotate the pinion about a horizontal axis or about a vertical axis.
When the driving mechanism for the auger boring machine that is described in U.S. application Ser. No. 10/886,808 was developed, the gear rack portion of each track section was cut from a thick steel plate using a cutting torch. In such method, a complex gear shape is cut on one side of the gear rack portion and a straight cut is made on the other side of the gear rack portion. Applicants have found that this method creates warping or “bowing” along the length of the gear rack portion of the track because heat stresses are concentrated on the “gear” side of the gear rack portion due to the considerable length of the cut and the time required to make it, as compared with the straight cut on the opposite side. The ends of the gear rack portion tend to remain in the plane of the plate from which it is cut and to bow or curve in a direction towards which the gear teeth point. Because the gear rack portion must be sufficiently thick and strong to support the auger boring machine during driving operation, it has little inherent flexibility. However, the gear rack must be straight in order for the drive mechanism of the auger boring machine to function properly. Consequently, if such a gear rack portion is warped or bowed along its length, it can only be straightened with much difficulty, if at all. Applicants have found that straightening such a gear rack typically requires several heat treatments and considerable time. It would be desirable, therefore, if a method could be developed that would mitigate and compensate for the harmful bowing or warping effect which has accompanied this method of manufacturing a track for the auger boring machine by cutting the gear rack portion from a steel plate using a torch or other heat source.